1. Field of the Invention
This invention relates to an oil filter disposed in a hydraulic circuit and adapted for cleaning oil therein.
2. Brief Description of the Prior Art
Ordinally, an injection molding machine, a large machine tool and the like have a hydraulic circuit for use in a hydraulic drive system including a cylinder for moving a table or a hydraulic circuit for lubricating a rotary driving unit. Such a hydraulic circuit is also utilized for automobiles or vehicles with internal combustion engines or diesel engines.
The hydraulic fluid used in such a hydraulic circuit is contaminated by very small particles of extraneous substances while circulating through the circuit. Therefore, an oil filter for filtering and cleaning the working fluid is provided in the circuit so as to maintain the lubricating performance and protect the machine.
The type of oil filter which has generally been used has a construction in which, as shown in FIG. 7, a filter element C formed of a roll of filter paper B is accommodated in a casing A capable of enduring pressure. This type of oil filter also has a port through which hydraulic fluid is introduced and a port (not shown) through which hydraulic fluid is recovered or discharged after filtration, these ports being disposed in a lower portion of the casing A. The hydraulic fluid circulating is thereby filtered through the hydraulic circuit of the machine during working.
Due to the effect of filtration of the hydraulic fluid, extraneous substances which had been contained in the hydraulic fluid stick to the filter element C so that the filter paper B finally loses its filtrating function. It is therefore necessary to change the filter element C at regular intervals by removing a cover (not shown) detachably attached to the casing A.
This operation of changing the filter element is performed after a hydraulic pump of the hydraulic circuit has been stopped. At this time, a certain quantity of hydraulic fluid remains in the casing A. When the cover is opened thereafter, the element C is soaked in the hydraulic fluid. There is therefore a possibility of hydraulic fluid flowing out of the casing A and contaminating the machine or the working floor. It is specifically necessary for an oil filter attached to a precision instrument to be provided with a dedicated leak prevention device if there is such a risk of hydraulic fluid leaking each time the element C is changed. Also the problem of leaking may complicate the changing operation itself.